Leukemia is predicted to cause over 23,000 deaths in 2012 in the U.S alone. Tumorigenesis in leukemia is often driven by constitutively active tyrosine kinases (e.g. Bcr-Abl, Tel-PDGFR, mutant Flt3), and much effort has been directed towards the development of agents that therapeutically inhibit these tyrosine kinases. Inhibitors of Bcr-Abl have dramatically improved the treatment of Bcr-Abl positive leukemias, yet resistance has required many patients to seek alternative therapies. Furthermore, inhibitors of Flt3, which is mutated in certain acute myeloid and lymphocytic leukemias, have produced only partial and/or short-lived responses in patients. When targeted therapies fail, standard chemotherapeutics are often the best treatment option for leukemia patients. Both standard and targeted chemotherapeutics work, at least in part, by inducing a form of programmed cell death called apoptosis. However, leukemogenic tyrosine kinases activate pro- survival pathways, allowing the cell to evade apoptosis, limiting the effectiveness of chemotherapeutics. It is our objective to identify molecules within the apoptotic pathway that can be targeted to prevent or reverse the chemoresistance caused by constitutively active tyrosine kinases in leukemia. We have shown that leukemogenic tyrosine kinases impede the formation of the apoptosome, a caspase-activating structure, nucleated by cytochrome c released from mitochondria in response to cell death stimuli. This failure of apoptosome formation contributes to the apoptotic resistance seen in leukemias. Apoptosome inhibition in leukemic cells is mediated by Hsp90, which binds to Apaf-1, a core constituent of the apoptosome; this binding occurs only when Hsp90 is hypophosphorylated at two key regulatory residues. We have determined that Hsp90 is dephosphorylated in leukemic cells by protein phosphatase 5 (PP5). PP5 is, in turn, regulated by acetylation at K144, and the hypoacetylated PP5, found specifically in leukemic cells, binds tightly to Hsp90 to catalyze its dephosphorylation. Our central hypothesis is that PP5, and the enzymes regulating its acetylation status, contribute significantly to chemoresistance in tyrosine kinase-induced leukemia. To test this hypothesis we propose (1) to identify the enzymes regulating PP5 K144 acetylation and determine their mechanism of regulation using leukemia cell models and (2) to assess the role of PP5 expression and K144 acetylation in leukemogenesis and in determining the chemosensitivity of tyrosine kinase-induced leukemias in vitro and in vivo. PP5 knock-out mice and cell lines will be used to test the effect of PP5 expression and acetylation status on responsiveness to clinically relevant chemotherapeutics. Although tyrosine kinase inhibitors have greatly increased survival among leukemia patients, it is now apparent that additional therapies are necessary to surmount both primary and acquired resistance. The proposed research seeks to identify therapeutic targets for the treatment of leukemia, which could be used to sensitize leukemia cells to chemotherapeutics, and ultimately lead to improved prognosis for leukemia patients.